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Thread: Safety, MPE for the sun ?

  1. #1
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    Question Safety, MPE for the sun ?

    Hi,

    I have a question for the experts in safety.
    I was researching solar power for my house this evening, and started calculating some stuff. But I have to be wrong at this somewhere. Because the results are nuts.

    Sunlight on a good day: 1367 Watt/m2 = 0.136W/cm2 = 136mW/cm2
    Your eye: 7mm. so the surface is 3.5mm x 3.5mm x 3.1415 = 38mm2

    Your eye gets 38mm2 x 1.36mW/mm2 = 51mW

    I told you the result is nuts...

    And I thought my 30mw laserpointer was dangerous

    Seriously, can somebody tell me where I am wrong ? I know that looking at the sun isnt very wise either, but 6.6 billion people do it everyday for a fraction of a second.

    here is the suns spectrum:
    http://www.nature.com/nphoton/journa...007.162-f5.jpg

  2. #2
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    At my company, we usually use 1KW/m^2 for the sun's irradiance. 1367 is a little hot...

    Still, your calcs look to be correct.

    Typically, when you're outside, your pupils are constricted to a very small opening (not 7mm diameter). So - the sun's input is considerably less than 51mw.

    When you're watching a lasershow, your pupils a dilated fully (7mm) because it's dark, and you get the 'full monty' so to speak...

    Of course, this whole dissertation may be FOS...

    Tim

  3. #3
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    pupil diameter indeed plays a greet role. And it is in fact the same factor as Bill's (pangolon) divergence rule. Double the divergence is 1/4th of the power.

    But just as for laser safety, I was pointing out the worst case scenario.
    (Imagine you are taking a nap on the beach, and suddenly you wake up with the sun directly in your face. You got 50mw right there.)

    Still, with a pupil diameter of 3mm (according to wikipedia this is on a bright day), you still get about 10mw.

    Now you can say. Hah, but we are talking about sunlight, which is not collimated and pretty divergent. Well, it appears that sunlight has a pretty good divergence: 0.5degrees, which is 8mrad. Almost as good as a 635red

  4. #4
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    Sunlight is not monochromatic. Sunlight is also not spatially coherent. Laser light is both of these things. So you have no chromatic abberations from the lens to worry about with laser light, thus you end up with a smaller spot size. Likewise, the waves from the sun do not all strike your eye in unison like the spacially coherent radiation of a laser does. So once again the damage is increased.

    Couple these two factors together, and you can see why laser light can be focused to a smaller spot (and one that causes more damage) that sunlight. However, your calculation also proves the point that lasers are *really* friggin' bright to begin with, and that's why safety is so important.

    Adam

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    Hi Adam,

    You can't just collect a light source into a 1 square centimeter detector and then run laser safety calculations on the result. It's about the size of the spot that the light source makes on your retina, and whether the "cooling" is effective or not. Yes, you hinted about a few of the factors, including coherence.

    In a paper written by David Sliney many years ago (he's one of the two biological physicists who did all of the initial research on which the MPE is based), he did some calculations about 5mW HeNe-based laser projectors that were popular at the time. He likened the cooling requirement of the 5mW HeNe to the sun, and in his write-up, prooved that they were safe to look at for a blink-reflex time (1/4 second).

    So you can think about the sun as having the same danger as a 5mW HeNe (and vice versa).

    Best regards,

    William Benner

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    Quote Originally Posted by Pangolin View Post
    You can't just collect a light source into a 1 square centimeter detector and then run laser safety calculations on the result.
    I know. I wasn't suggesting that at all.

    Highvolt's question was: if 51 mw of solar radiation doesn't hurt you (for a brief exposure), why is 30 mw of laser radiation dangerous?

    I poinited out that the factors he wasn't taking into consideration are exactly those factors that make laser light vastly different (and more dangerous) than sunlight.

    He correctly deduced that divergence isn't the issue, but forgot about coherence (both spatial and frequency). This is why the MPE for laser light is lower than it would be for sunlight.
    It's about the size of the spot that the light source makes on your retina, and whether the "cooling" is effective or not.
    Correct - and the fact that the spot is smaller with laser radiation is due to the fact that it's monochromatic and spatially coherent. I had hoped to avoid diving into the physics behind chromatic abberations and diffraction-limited spots, but yeah - that's where I was headed.
    In a paper written by David Sliney many years ago (he's one of the two biological physicists who did all of the initial research on which the MPE is based), he did some calculations about 5mW HeNe-based laser projectors that were popular at the time. He likened the cooling requirement of the 5mW HeNe to the sun, and in his write-up, prooved that they were safe to look at for a blink-reflex time (1/4 second).
    I've seen another comparison that suggests that the sun is actually much weaker than a 5 mw laser in terms of power density. However, that calculation assumes a diffraction-limited spot, which is a bit unrealistic considering the imperfections of the lens in the human eye.

    Nevertheless, the reason the sun doesn't cause damage for a 1/4 second exposure when a laser can cause damage (even at the same average power level) is because laser light is not the same thing as sunlight, for the reasons outlined above. (And yes, because of those differences it gets focused to a much smaller spot, creating localized heating that can't be removed fast enough by the relatively widely spaced blood vessels, thus causing damage to the retina...)

    Adam
    Last edited by buffo; 05-13-2008 at 08:30. Reason: Typo

  7. #7
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    Thx Adam, I read some stuff from Sam's laser FAQ, but I haven't read that section before. Very interesting stuff!

    Highvolt's question was: if 51 mw of solar radiation doesn't hurt you (for a brief exposure), why is 30 mw of laser radiation dangerous?
    That's not really what I've said. (or was trying to say).
    I am not trying to compare a 30mw pointer with the sun here. I know that the pointer is dangerous, I just realized that after the calculations, the sun seemed to be superdangerous.
    Maybe the sarcasm was lost during my dutch to english translations We get that a lot in our company as well (dutch, french, german and English colleagues... big communication issues sometimes)

    So, if I understand it correctly. While I calculated the amount of mW entering the eye, the big difference lies in the spot it focuses on.

    Does this also mean that Bill's saying of "divergence is the key for safety" means that the safety factor not only comes by less power entering the eye, but the focused spot is bigger with a divergent beam ?
    (divergent beam = non parallel waves = bigger spot on retina)

    Oh, and another thing, if a divergent beam is safer, then everyone should like the beamspecs of a 635nm red no ? In fact, they should make green and blue lasers with >5mrad
    (tip: last paragraph can contain some sarcasm )

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    Quote Originally Posted by highvolt View Post
    So, if I understand it correctly. While I calculated the amount of mW entering the eye, the big difference lies in the spot it focuses on.
    Correct. A laser can be focused to a very small spot. Sunlight can't. (You can demonstrate this yourself with a magnifying glass.)
    Does this also mean that Bill's saying of "divergence is the key for safety" means that the safety factor not only comes by less power entering the eye, but the focused spot is bigger with a divergent beam ?
    (divergent beam = non parallel waves = bigger spot on retina)
    Not exactly. Bill is referring to divergence being the key because it limits the power entering the eye. However, your eyes can still focus a divergent beam to a small spot, it just requires a little more work from the lens in your eye.
    Oh, and another thing, if a divergent beam is safer, then everyone should like the beamspecs of a 635nm red no ? In fact, they should make green and blue lasers with >5mrad
    (tip: last paragraph can contain some sarcasm )
    Haha! But no, as we see above, divergence doesn't prevent you from focusing the beam to a small spot.

    Actually, there is good reason to use 635 nm red vs 660 nm red though: The 635 nm red will appear brighter to the human eye, so you don't need to use as much power. (100 mw in a divergent beam is safer than 400 mw in a divergent beam.)

    Adam

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